Electronic products recently have more and more miniaturized and lighter in weight. In accordance with this, various electronic parts are required to be downsized. Based on the ground that flexible printed wiring boards have the advantage of flexibility and light weight, their demand greatly increases.
With respect to the materials used, coverlay films can be divided into the following three types: photosensitive coverlay film (first type), non-photosensitive coverlay film (second type) and thermal plastic coverlay film (third type). The photosensitive coverlay film (first type) also can be divided into polyimide based coverlay film (PT based coverlay film) and non-polyimide based coverlay film (non-PI based coverlay film). Since the non-PI based coverlay film has poor heat resistance, higher coefficient of thermal expansion (CTE), etc., hence its applications are more restricted. The non-photosensitive coverlay film (second type) is more complex in process than is the photosensitive coverlay film (first type), and thus the practicability of the non-photosensitive coverlay film is inferior to that of the photosensitive coverlay film. As for the thermal plastic coverlay film, a holes-formation operation as a post-process is needed, and therefore in the respect of the convenience, the thermal plastic coverlay film is inferior to the photosensitive coverlay film.
U.S. Pat. No. 6,605,353 has disclosed an epoxy-modified photosensitive polyimide. The epoxy-modified photosensitive polyimide is prepared by reacting an epoxide with an acid. However, based on the ground that OH functional groups generated after ring-opening reaction can then react with epoxy groups, the epoxy-modified photosensitive polyimide accordingly has the problems of storage-stability and heat-stability.
In order to improve the heat resistance and process convenience, there is a desire to find a novel photosensitive polyimide which can overcome the above-mentioned problems.